Amorphous nano-scale carbon tube and production method therefor

ABSTRACT

Straight, nano-scale-order amorphous carbon tubes having a long-term stable ability for storing various kinds of gases and being stable in shape, and a novel process for producing said carbon tubes with high purity, high yield and high mass-productivity are provided. 
     The amorphous nano-scale carbon tubes are prepared by subjecting a heat-decomposable resin having a decomposition temperature of 200 to 900° C. to an excitation treatment in the presence of a metal powder and/or a metal salt, or by subjecting a carbon material containing —C≡C— and/or ═C═ to a heat-treatment at 3000° C. or lower.

TECHNICAL FIELD

The present invention relates to amorphous nano-scale carbon tubes eachof which comprises carbon as a main framework and has a diameter of 0.1to 1000 nm and a hollow fiber shape; a carbon material containing theamorphous nano-scale carbon tubes; and a method for producing theamorphous nano-scale carbon tubes or the carbon material.

BACKGROUND ART

Conventionally, carbon fibers are produced by forming a startingmaterial such as pitch or polyacrylonitrile into fibers whilemaintaining the main chain framework of the starting material. This typeof method is incapable of producing a product wherein the nano-scalemolecules are controlled.

Carbon nanotubes (hereinafter referred to as “CNTs”), which have beenattracting attention recently, can be roughly regarded as fiberscontrolled at the molecular level. CNTs are produced by the carbon arcmethod, sputtering, laser beam irradiation or like technique, usinggraphite or like starting carbon material in the presence of a metalcatalyst. However, these techniques chiefly produce carbon nanotubeshaving a graphitic structure. One example of a carbon nanotube of such astructure is a carbon filament comprising a graphitic core surrounded byirregular pyrrolytic carbon (Oberlin, Endo, Koyama; Carbon 14, 133(1976)).

Also, Japanese Examined Patent Publications No. 1991-64606 and No.1991-77288 and other publications disclose carbon fibrils eachcomprising a graphitic outer portion and an inner core comprisingirregular carbon atoms. In the disclosed techniques, however, it ispractically difficult to control straightness of molecules or shapefactors such as diameter and length.

Further, attempts have been made to produce CNTs using acetylene or likegaseous hydrocarbon as a starting material in the presence of a catalystsuch as iron. However, also in these techniques, it is practicallydifficult to control straightness of molecules, diameter, length and thelike while maintaining the graphitic structure. Stated specifically, inall of these techniques, the starting materials are excited into agas-phase activated carbon state and then allowed to form CNTs duringthe process of recombination. Therefore, it is extremely difficult tocontrol the reaction parameters such as the amount of the startingmaterial, resulting in production of CNTs having a graphitic structureand varying widely in shape. The products obtained by these techniqueshave a degree of graphitization (crystallinity) of at least 5%, and 50%to 100% in most cases. Further, the end each of the CNTs is closed witha cap and CNTs often contain a metal at the tips.

One report points out the existence of carbon nanotubes having anamorphous structure, as a precursor of graphitic carbon nanotubes.However, the existence of the carbon nanotubes with an amorphousstructure is merely presumed from the existence of carbon tubes with anamorphous structure which are observed, through TEM, among the graphiticcarbon nanotubes. Moreover, the amorphous carbon tubes are reported tobe an intermediate product temporarily formed in the process of formingthe graphitic carbon tubes. Thus, the selective synthesis method or useof the amorphous carbon nanotubes have not been elucidated yet (WenlowWang et al: Electrochemical Society Proceedings Volume 97-14,814(1997)).

As discussed above, it is practically difficult for the conventionaltechniques to control the crystal structure, molecular straightness,diameter, length, end structure and the like of the CNTs. In particular,the CNTs have a substantially graphitic structure, and thus have lowdegree of freedom of structural control. Moreover, the conventionaltechniques have the problem that amorphous carbon formed as a by-productcontaminates the graphite product and makes purification extremelydifficult.

Regarding the properties of CNTs, it has been reported that CNTs arelikely to adsorb a hydrogen gas densely by their capillary action (A. C.Dillon et al: Nature, 386, 377(1997)). Further, U.S. Pat. No. 5,653,951states that solid layered nanostructures (graphite nanofibers) arecapable of chemisorbing a large amount of hydrogen into the intersticesof the graphite layers.

However, reports on these known materials merely point out possibleperformance characteristics of the materials on a research anddevelopment level. This is because the prior art techniques have anumber of problems such as difficulties in material synthesis processand in control of structure and shape that affect the materialstability, lack of mass-productivity, and the like. Moreover, thegraphite nanofibers do not have sufficient durability for repetitiveuse, since the distance between the graphite layers expands as thefibers adsorb hydrogen. Accordingly, there remain a number of problemsto be solved before putting the known materials into practical use.

OBJECTS OF THE INVENTION

The main object of the present invention is to provide amorphousnano-scale carbon tubes in hollow fiber shape, which are excellent inmechanical, electronic and chemical properties and comprise carbonhaving a structure and shape controlled at a nano-scale level; and anovel method that enables industrial production of such amorphousnano-scale carbon tubes in high purity, high yield and highmass-productivity.

A further object of the present invention is to provide a gas-storingmaterial comprising an amorphous nano-scale carbon tube, which has astable high capacity for storing a variety of gases and possessesexcellent durability; and a method for storing a gas utilizing the gasstoring material.

DISCLOSURE OF THE INVENTION

The present inventors conducted extensive research in view of the abovestatus of the prior art, and obtained the following findings:

(1) The wall each of the amorphous nano-scale carbon tubes has amorphousstructure consisting of hexagonal carbon layers oriented in alldirections. The spacing between the hexagonal carbon layers impartsflexibility to the carbon tubes, and enables the carbon tubes to expand,to absorb gas molecules and to disperse the gas molecules within eachtube. Amorphous nano-scale carbon tubes each comprising an aggregate ofthe hexagonal carbon layers are higher in gas-storing capability anddurability than CNTs having a non-amorphous structure such as agraphitic structure.

(2) Amorphous nano-scale carbon tubes, if having open ends, require nocomplex process for opening the ends. Also, amorphous nano-scale carbontubes having flat ends possess a relatively large structural strain atthe ends, and are therefore easy to open at the ends.

(3) Further, amorphous nano-scale carbon tubes having a straight shapeare advantageous in dense packing of the material and gas dispersionwithin the material.

(4) Furthermore, amorphous nano-scale carbon tubes having a flexible orelastic amorphous structure which can absorb external force, are usefulfrom the standpoint of sliding property, abrasion resistivity and thelike.

(4) When amorphous nano-scale carbon tubes are produced by placing aheat decomposable resin, which decomposes at a specific temperature,into an excited state in the presence of a catalyst comprising a metalpowder and/or a metal salt, amorphous nano-scale carbon tubes with theabove specific structure and shape can be produced in high purity, highyield and high mass-productivity.

(5) The amorphous nano-scale carbon tubes obtained by the above methodare particularly suitable as a gas-storing material, a sliding material,an abrasion-resistant material or the like.

The present inventors conducted further research based on thesefindings, and consequently developed straight, stable and nano-odernano-scale carbon tubes having an amorphous structure, and a method forproducing the nano-scale carbon tubes in high purity, high yield andhigh mass-productivity.

The present invention provides the following amorphous nano-scale carbontubes or carbon materials comprising the amorphous nano-scale carbontubes, and methods for producing the same.

1. Nano-scale carbon tubes each containing a main framework whichcomprises carbon, and each having a diameter of 0.1 to 1000 nm and anamorphous structure.

2. The nano-scale carbon tubes according to item 1, each of whichcomprises hexagonal carbon layers each having a dimension in the planardirection that is smaller than the diameter of the carbon tube, asdetermined from a transmission electron microscope image.

3. The nano-scale carbon tubes according to item 1 or 2, each of whichhas an interlayer spacing (002) between hexagonal carbon layers of atleast 3.54 Å, a diffraction angle (2θ) of 25.1 degrees or less, and a 2θband half-width of at least 3.2 degrees, as determined with adiffractometer by an X-ray diffraction method (incident X-ray: CuKα).

4. The amorphous nano-scale carbon tubes according to any one of items 1to 3, each of which has a straight shape.

5. The amorphous nano-scale carbon tubes according to any one of items 1to 4, each of which has a hollow cylindrical shape or a hollowrectangular prism shape.

6. The amorphous nano-scale carbon tubes according to any one of items 1to 5, each of which has at least one open or flat end.

7. The amorphous nano-scale carbon tubes according to any one of theitems 1 to 6, which are formed on a substrate, a particle or a porousmaterial.

8. A gas-storing material comprising an amorphous carbonaceous materialcontaining the amorphous nano-scale carbon tubes according to any one ofitems 1 to 7.

9. The gas-storing material according to item 8, which contains at leastone of a metal salt and a metal.

10. The gas-storing material according to item 9, wherein the metal saltand the metal are selected from the group consisting of iron, cobalt,nickel, copper, platinum, palladium, rubidium, strontium, cesium,vanadium, manganese, aluminum, silver, lithium, potassium, sodium,magnesium, hydrogen-occluding alloys and metal complexes.

11. A method for storing a gas, wherein a gas is stored using thegas-storing material according to any one of items 8 to 10.

12. The method according to item 11, wherein the gas to be stored ishydrogen, methane, helium, neon, xenon, krypton or carbon dioxide.

13. A method for producing a carbon material containing the amorphousnano-scale carbon tubes according to any one of items 1 to 7, the methodcomprising subjecting a heat decomposable resin having a decompositiontemperature of 200 to 900° C. to excitation treatment in the presence ofa catalyst comprising a metal powder and/or a metal salt.

14. The method for producing said carbon material containing theamorphous nano-scale carbon tubes according to item 13, wherein thecatalyst comprising a metal powder and/or a metal salt is at least onemember selected from the group consisting of alkaline earth metals,iron, cobalt, nickel, chromium and their salts.

15. The method of producing said carbon material containing theamorphous nano-scale carbon tubes according to item 13 or 14, whereinthe excitation treatment of the heat decomposable resin is carried outby heat treatment in an inert gas at a temperature of 300 to 3000° C.

16. The method for producing said carbon material containing theamorphous nano-scale carbon tubes according to item 13 or 14, whereinthe excitation treatment of the heat decomposable resin is carried outby light irradiation treatment in an inert gas at a temperature of roomtemperature to 3000° C.

17. The method for producing said carbon material containing theamorphous nano-scale carbon tubes according to item 13 or 14, whereinthe excitation treatment of the heat decomposable resin is carried outby plasma treatment in an inert gas at a temperature of room temperatureto 3000° C.

18. The method for producing said carbon material containing theamorphous nano-scale carbon tubes according to item 13 or 14, whereinthe excitation treatment of the heat decomposable resin is carried outby electron beam irradiation treatment in an inert gas at a temperatureof room temperature to 3000° C.

19. The method for producing said carbon material containing theamorphous nano-scale carbon tubes according to item 13 or 14, whereinthe excitation treatment of the heat decomposable resin is carried outby ion beam irradiation treatment in an inert gas at a temperature ofroom temperature to 3000° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a plasma excitation apparatus usedin the working examples of the present invention.

FIG. 2 is an X-ray diffraction chart of amorphous nano-scale carbontubes obtained in Example 1.

FIG. 3-A is a TEM photograph showing an amorphous nano-scale carbon tubeobtained in Example 3.

FIG. 3-B is a TEM photograph showing a further enlarged image of theamorphous nano-scale carbon tube shown in FIG. 3-A.

FIG. 4 is an X-ray diffraction chart of amorphous nano-scale carbontubes obtained in Example 3.

FIG. 5 is an X-ray diffraction chart of amorphous nano-scale carbontubes obtained in Example 9.

FIG. 6-A is a TEM photograph showing an amorphous nano-scale carbon tubeobtained in Example 18.

FIG. 6-B is a TEM photograph showing a further enlarged image of theamorphous nano-scale carbon tube shown in FIG. 6-A.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, the heat decomposable resin for useas a starting material is not particularly limited, as far as it has adecomposition temperature of about 200 to 900° C. (preferably about 400to 900° C.). Specific examples of such resins includepolytetrafluoroethylene (PTFE), polyvinylidene chloride, polyvinylidenefluoride, polyethylene (PE), polyvinyl alcohol (PVA), polyimide,polyacrylonitrile and the like. Of the above heat decomposable resins,PTFE, PE, PVA, polyacrylonitrile and the like are preferred.

The heat decomposable resin as the starting material may be in anyforms, such as film or sheet, powder, mass or the like. For example, theheat decomposable resin may be applied to or mounted on a substrate, andthen subjected to excitation treatment under suitable conditions,whereby a carbon material comprising a thin layer of amorphousnano-scale carbon tubes formed on the substrate is obtained.

Catalysts usable for excitation treatment of the heat decomposable resininclude, for example, iron, cobalt, nickel, chromium, magnesium and likemetals; halides, complexes and like salts of these metals. Of the abovecatalysts, iron chloride is preferable. The particle size of thecatalyst is not particularly limited, but is usually 5 mm or less, morepreferably 100 μm or less.

Excitation of the starting heat decomposable resin is carried out withthe catalyst in contact with the starting material (for example, withthe catalyst particles applied to the surface of a film- or sheet-formstarting material, or with a powdery starting material mixed withcatalyst particles) in an inert atmosphere (in an atmosphere of inertgas such as Ar, He or N₂; at a pressure ranging from an elevatedpressure to reduced pressure, preferably at a pressure not higher than 2atms, more preferably at reduced pressure of about 400 torr or lower:under reduced pressure in the presence of an inert gas, and the like).The amount of the catalyst relative to the starting heat decomposableresin may greatly vary depending on the form and kind of the startingmaterial, the kind and particle size of the catalyst, and the like.Usually, however, the catalyst is used in an amount of about 10 to1/1000 times, preferably 1/10 to 1/50 times, the weight of the startingmaterial.

The excitation of the starting material can be carried out by variousmeans such as heat treatment, light irradiation treatment, plasmatreatment or the like.

For excitation by heat treatment, the starting material is heated at atemperature which is not higher than 3000° C., preferably about 300 to2000° C., more preferably about 450 to 1800° C., and which is not lowerthan the heat decomposition temperature of the starting material.

Excitation by light irradiation is carried out at a temperature which iswithin a range from room temperature to about 3000° C., and which is notlower than the heat decomposition temperature of the starting material.Usually a laser beam having a wavelength up to approximately 1200 nm,preferably a wavelength of about 150 to 1200 nm, is used for lightirradiation treatment. Any types of laser beams conventionally used canbe employed without limitation. Usable lasers include, for example,Nd:YAG laser, Ti:Sa laser, Dye laser, Dye+SHG laser, Ar⁺ laser and Kr⁺laser.

Excitation by plazma treatment is carried out at a temperature which iswithin a range from room temperature to about 3000° C. and which is notlower than the heat decomposition temperature of the starting material.The process for plasma irradiation is not limited, and may compriseplacing the starting material in an inert gas atmosphere or a reducinggas atmosphere, and contacting the starting material with a high-energyplasma stream to thereby obtain the desired product. In order togenerate the plasma stream, an electromagnetic excitation source isused. The conditions for plasma generation can be suitably selectedaccording to the kind of the gas, gas pressure, excitation voltage,excitation current, excitation power source frequency, electrode shape,etc.

Some gases are difficult to convert into a plasma state owing to theircharacteristics, but they can be converted into a plasma state byinputting an increased amount of excitation electromagnetic power.Examples of gases usable for plasma treatment in the invention includeAr, He, Kr, N₂ and like inert gases, hydrogen or like reducing gases,and mixtures of these gases. Among the above gases, Ar, He and the likeare more preferred.

The gas pressure for the plasma treatment needs to be selected inrelation to the input of the excitation electromagnetic power. With anincrease in gas pressure, the number of gas molecules increases, and alarge amount of energy is needed for excitation of individual gasmolecules. Thus, a great amount of excitation electromagnetic power isnecessary. For example, plasma generation is possible even at a gaspressure of 10 atms or more, but necessitates a large electric powersource and therefore involves an extremely high facility cost. Also, ahigh excitation voltage and a high excitation current enable generationof a large amount of plasma particles, but an excessively high electricenergy input or an excessively low pressure makes it difficult for theelectromagnetic energy to smoothly transmit to the gas, and causesdischarge between the electrodes, failing to generate a sufficientamount of plasma particles. On the other hand, when the gas pressure islow, a plasma is generated with a relatively low input ofelectromagnetic excitation power, but an excessively low pressureresults in insufficient amount of plasma. In view of these factors, itis preferable that the gas pressure at the time of plasma generation is10⁻² torr to atmospheric pressure.

The electromagnetic power may be of a direct current or an alternatingcurrent, and the material and shape of the electrodes can be selectedaccording to the form of the electromagnetic power to be input. As thealternating current, a low frequency current of about 50 to 60 Hz orabout 1 to 10 kHz, or a high frequency current of about 10 MHz toseveral GHz is usually employed. As an industrial high frequencycurrent, a current of 13.56 MHz, 40 MHz, 915 MHz or 2.45 GHz isgenerally employed. Materials usually used for the electrodes includestainless steel, aluminum and its alloys, common steel and the like. Theshape of the electrodes is selected from capacitively coupled type,parallel plate type, hollow cathode type, coil type, etc.

A desired plasma can be generated easily at low cost, for example, byinputting a several hundred watts of electric power to coil typeelectrodes using a power source with a high frequency of 13.56 MHz, inan inert gas such as Ar, He, Kr or N₂, a reducing gas such as hydrogen,or a mixture of these gases, each having been adjusted to a reducedpressure of 1×10⁻³ torr to several hundred torr.

When the starting material is subjected to electron beam irradiation,the irradiation is carried out under reduced pressure of usually about10⁻² to 10⁻⁷ torr (preferably about 10⁻³ to 10⁻⁵ torr) with anacceleration voltage of about 1 to 2000 kV (preferably about 50 to 1000kV), while maintaining the starting material at a temperature of roomtemperature to 3000° C.

When the starting material is subjected to ion beam irradiation, thestarting material is placed in a reduced pressure chamber (in which thepressure is reduced to usually about 100 to 10⁻⁷ torr, preferably about10⁻¹ to 10⁻⁵ torr), and irradiated with ionized He ions or Ar ions, withan acceleration voltage of about 100 V to 10 kV (preferably about 200 Vto 1 kV) and an ion current of about 0.01 to 100 mA/cm² (preferablyabout 0.1 to 10 mA/cm²).

Amorphous nano-scale carbon tubes or a carbon material containingamorphous nano-scale carbon tubes can also be synthesized byheat-treating a carbon material containing reactive —C≡C— and/or ═C═.The heat treatment of the material is carried out in an inert atmosphere(in an inert gas such as Ar, He or N₂; at a pressure which is lower thanatmospheric pressure, preferably under reduced pressure of about 400torr or lower: under reduced pressure in the presence of an inert gas,or the like), at a temperature of 3000° C. or lower, preferably about300 to 2000° C., more preferably 450 to 1800° C. The carbon materialcontaining reactive —C≡C— and/or ═C═ is highly reactive owing to thetriple bond in its molecule, and therefore can be easily made intoamorphous nano-scale carbon tubes or a carbon material containingamorphous nano-scale carbon tubes.

The term “carbon material containing —C≡C— and/or ═C═” as used hereinencompasses a material comprising at least one of polyyne and cumulene,a material comprising at least one of —C≡C— and ═C═ bonds, a materialpartially comprising at least one or polyyne and cumulene, a materialpartially comprising at least one of —C≡C— and ═C═ bonds, and the like.The above term also encompasses a material comprising any of the abovematerials and a metal powder and/or a metal salt dispersed in thematerials.

The “carbon material containing —C≡C— and/or ═C═” for use in the presentinvention and the process for its synthesis are both known. For example,polyyne having reactive triple bonds is disclosed in Japanese UnexaminedPatent Publications No. 1991-44852 and No. 1998-199726; M. Kijima et al,Synthetic Metals, 86(1997), 2279; and so on.

Further, a carbon material containing polyyne is disclosed in J.Kansther et al, Macromolecules, 28(1975); L. Kavan et al, Carbon, 32(1994), 1533; and so on.

The above publications also described a carbon material containing —C≡C—and/or ═C═.

The amorphous nano-scale carbon tubes according to the present inventionare carbon nanotubes of nono-scale, having an amorphous structure, ahollow straight shape, and highly controlled pores. Each of the tubesusually has a shape of cylinder or rectangular prism, and most of thetubes have at least one uncapped (open) end. In the case where tubeswith closed ends are present, most of these tubes have flat ends.

The amorphous nano-scale carbon tubes of the invention each has adiameter of usually about 0.1 to 1000 nm, preferably about 1 to 200 nm,more preferably about 1 to 100 nm. Each tube has a length twice as longas, more preferably 5 times as long as, its diameter.

“Amorphous structure” means a carbonaceous structure consisting ofdisordered hexagonal carbon layers, which is different from a graphiticstructure consisting of continuous carbon layers of regularly disposedcarbon atoms. In view of an image through a transmission electronmicroscope, which is typical analytical means, an amorphous nano-scalecarbon tube according to the invention can be defined as a carbon tubein which the dimension in the planar direction of the hexagonal carbonlayers is smaller than the diameter of the carbon tube.

Generally, amorphous carbon causes no X-ray diffraction but shows abroad reflection.

In a graphitic structure, hexagonal carbon layers are regularly stackedon one another, so that spacing between the hexagonal carbon layers(d₀₀₂) is narrow. Accordingly, the broad reflection shifts towards thehigh-angle side (2θ) and gradually narrows (has a smaller half-width ofthe 2θ band). As the result, the reflection can be observed as a d₀₀₂diffraction band (d₀₀₂=3.354 Å when the layers are regularly stacked onone another with a graphitic configuration).

In contrast, an amorphous structure generally does not cause X-raydiffraction as described above, but partially shows very weak coherentscattering. As determined by an X-ray diffraction method (incidentX-ray: CuKα) with a diffractometer, the theoretical crystallographiccharacteristics of the amorphous nano-scale carbon tubes of theinvention are defined as follows: the spacing between hexagonal carbonlayers (d₀₀₂) is at least 3.54 Å, preferably at least 3.7 Å; thediffraction angle (2θ) is 25.1 degrees or less, preferably 24.1 degreesor less; and the 2θ band half-width is at least 3.2 degrees, preferablyat least 7.0 degrees.

It is preferable that nano-scale carbon tubes having such amorphousstructure (amorphous carbon) account for more than 95%, preferably atleast 99%, of the amorphous nano-scale carbon tubes of the invention asa whole.

The term “straight” which is one of the terms describing shape of theamorphous nano-scale carbon tubes of the invention, is defined as havingthe following property: When the length of an amorphous nano-scalecarbon tube as determined from a TEM image is taken as L and the lengthof the carbon tube as stretched is taken as L_(O), the value L/L₀ is atleast 0.9.

It is preferable that such straight amorphous nano-scale carbon tubesaccount for at least 90%, preferably at least 95%, of the amorphousnano-scale carbon tubes of the invention as a whole.

It was initially reported that conventional CNTs were likely to store agas such as hydrogen, but the gas-storing capacity has not been directlyconfirmed. Therefore, conventional CNTs are not applicable to practicaluse as a gas-storing material.

In contrast, the amorphous nano-scale carbon tubes of the invention havea straight shape and a highly controlled pore size. Further, the carbontubes of the invention can disperse a gas also within the spacingbetween the hexagonal carbon layers, and thereby absorb the expansioncaused by physisorption of the gas into the hollow portion, thus havinga unique feature of possessing high durability. Therefore, the amorphousnano-scale carbon tubes of the invention are highly useful as agas-storing material. Moreover, the pore size of the material of theinvention can be controlled at a molecular level, so that the materialcan be used as a material for selectively adsorbing/storing a specificcompound, or as a molecular sieve.

Furthermore, many of the amorphous nano-scale carbon tubes of theinvention have at least one open end, obviating the complicating processfor opening the ends. Further, most of the carbon tubes of the inventionwhich have closed ends have flat ends and have a relatively highstructural strain at the ends. The ends of carbon tubes with flat endsare also easy to open.

Moreover, since the amorphous nano-scale carbon tubes of the inventionhave a straight shape, they can be densely packed for gas storing andare advantageous for dispersion of a gas within the material.

Furthermore, the carbon tubes of the invention and a carbon materialcontaining the carbon tubes have flexibility to absorb an externalforth, and thus are useful as a sliding material, a wear resistivematerial or the like.

The amorphous nano-scale carbon tubes of the invention or the carbonmaterial containing the carbon tubes are expected to have a highcapacitance owing to their properties, when used as an anode for lithiumsecondary batteries.

The amorphous nano-scale carbon tubes of the invention or the carbonmaterial containing the carbon tubes are also useful as a semiconductormaterial, semiconductor fibrils, an electron emitting material or thelike.

EFFECTS OF THE INVENTION

The present invention accomplishes the following remarkable effects:

-   -   (a) Nano-scale carbon tubes being straight, stable and        nano-order and having an amorphous structure can be obtained,        which are useful as a novel gas-storing material having        excellent durability and a high capacity to stably store a        variety of gases.    -   (b) Amorphous nano-scale carbon tubes which were hitherto not        known can be obtained through a synthesis mechanism different        from known techniques by using, as a starting material, a heat        decomposable resin or a carbon material containing reactive        —C≡C— and/or ═C═.    -   (c) Amorphous nano-scale carbon tubes with a reduced amount of        impurity, or a carbon material containing the carbon tubes can        be produced in a high yield. Therefore, desired nano-scale        carbon tubes can be purified and obtained easily, realizing high        mass-productivity on an industrial scale.    -   (d) The amorphous nano-scale carbon tubes have a hollow straight        shape, and can be formed into a thin layer on a substrate.        Accordingly, the carbon tubes are extremely useful as a material        for electronic devices.    -   (e) The amorphous nano-scale carbon tubes or a carbon material        containing the carbon tubes are useful as a gas-storing        material, a high elasticity material, a high strength material,        a wear resistive material, an electron beam emitter, a highly        directional X-ray source, a soft X-ray source, a one-dimensional        conductor, a high heat-conductive material, other materials for        electronic devices, or the like.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

The following Examples and Reference Examples are provided to illustratethe features of the present invention in further detail. However, thepresent invention is not limited to these Examples.

EXAMPLE 1

10 mg of an anhydrous iron chloride powder (having a particle size notgreater than 500 μm) was dusted uniformly over a PTEE film (60 μm×10mm×10 mm), and the film was then excited by plasma. FIG. 1 shows theoutline of a thin film forming apparatus 1 used. It is a matter ofcourse that an apparatus having a structure other than that shown inFIG. 1 can be used in the present invention.

The thin film forming apparatus 1 shown in FIG. 1 comprises a reactor 4equipped with a material gas inlet 2 and a gas discharge port 3, a firstelectrode 5 made of a helical wire and provided in the reactor 4 in aninsulated manner, and further comprises a support member 8 electricallycontacted with an object 6 and supporting the object 6 within a plasmageneration area 7 surrounded by the first electrode 5; the supportmember 8, in combination with an internal wall 4 a of the reactor 4,being electrically grounded to serve as a second electrode 2.Consequently, application of a high frequency current to the firstelectrode 5 induces a high frequency electric field in the plasmageneration area 7, to thereby generate a so-called inductively coupledplasma.

The size of the helical first electrode 1 may be varied depending on thesize of the object 6 held inside the helix. The helix has a diameter ofabout 1.5 to 10 times the overall dimension in the radial direction ofthe object 6, and has a height, in the axial direction perpendicular tothe radial direction of the helix, of about 1.5 to 3 times the overalldimension in the axial direction of the object 6. Loops of the helicalelectrode 5, adjacent in the axial direction, are out of contact withone another and have intervals of preferably 1 to 10 mm.

In operation of the apparatus, a material gas supplied from the gasinlet 2 into the reactor 4 is introduced into the plasma generation area7 through clearances between the loops of the first electrode 5. Apredetermined high frequency current is passed through the firstelectrode 5 in the following manner: While one end of the firstelectrode 5 is open, a high frequency power source 11 with a maximumoutput of 10 kW is connected to the other end of the first electrode 5via a matching circuit 10 provided outside the reactor 4, to apply ahigh frequency voltage of 13.56 MHz. A vacuum pump 12 is connected tothe gas discharge port 3 to reduce the pressure in the reactor 4 to apredetermined reaction pressure.

Accordingly, when the high frequency voltage is applied to the firstelectrode 5 in the above manner, the first electrode 5 serves as acathode electrode so as to generate another high frequency electricfield (termed a capacitively coupled high frequency electric field)between the electrodes 5 and 9, in addition to the induction highfrequency electric field. The inductively coupled plasma converts thematerial gas introduced into the plasma generation area 7, tohigh-energy active chemical species. The active chemical species, ascharged particles, receive an electromagnetic force from the above twotypes of high frequency electric fields and from a high frequencymagnetic field induced by the high frequency current, thereby excitingthe object 6 with the resulting plasma.

The conditions for plasma excitation were as follows.

Atmosphere Ar Internal Pressure 0.01 torr Electric power input 300 W RFfrequency 13.56 MHz

After completion of the reaction, formation of amorphous nano-scalecarbon tubes (diameter: 10 to 60 nm, length: 5 to 6 μm) was confirmed bySEM and X-ray diffraction.

FIG. 2 is an X-ray diffraction chart of the amorphous nano-scale carbontubes obtained. FIG. 2 shows that the diffraction angle (2θ) was 19.1degrees; that the spacing between the hexagonal carbon layers (d₀₀₂)calculated from the diffraction angle was 4.6 Å; and that the half-widthof 2θ band was 8.1 degrees.

EXAMPLE 2

10 mg of an anhydrous iron chloride powder (having a particle size notgreater than 500 μm) was dusted uniformly over a PTFE film (60 μm×10mm×10 mm), and the film was placed in a vacuum furnace. The pressure inthe furnace was reduced to 0.1 Pa, and then the film was irradiated witha laser beam.

The conditions for laser beam irradiation were as follows.

Atmosphere He Internal pressure 500 torr Temperature 800° C. Input laserbeam wavelength 248 nm Input laser beam power density 17 mJ/pulse/cm²Number of cycles of input 1 Hz laser beam Period of irradiation with 30minutes the input laser beam

After completion of the reaction, formation of amorphous nano-scalecarbon tubes was confirmed by SEM and X-ray diffraction. The productobtained in this Example had a diameter and length approximately equalto that of Example 2.

EXAMPLE 3

10 mg of an anhydrous iron chloride powder (having a particle size notgreater than 500 μm) was dusted uniformly over a PTFE film (60 μm×10mm×10 mm). The film was placed in a vacuum furnace. The furnace waspurged with nitrogen 3 times, and the pressure in the furnace wasreduced to 3 Pa. Then, the film was vacuum baked at 900° C. for 10minutes.

Formation of amorphous nano-scale carbon tubes was observed by SEM andX-ray diffraction. The product obtained in this Example had a diameterand length approximately equal to that of Example 1.

FIG. 3-A and FIG. 3-B are TEM photographs of an amorphous nano-scalecarbon tube obtained. FIG. 4 is an X-ray diffraction chart of the carbontubes thus obtained.

TEM observation revealed that, in each of the obtained amorphousnano-scale carbon tubes, the dimension in the planar direction of thehexagonal carbon layers is smaller than the diameter of the carbon tube.The X-ray diffraction angle (2θ) was 18.9 degrees; the spacing betweenthe hexagonal carbon layers (d₀₀₂) calculated from the diffraction anglewas 4.7 Å; and the 2θ band half-width was 8.2 degrees.

EXAMPLES 4 TO 10

Amorphous nano-scale carbon tubes were formed in the same manner as inExample 3 except that the vacuum conditions, temperature and type ofcatalyst (only in Example 6) were changed as shown in Table 1. Theproduct obtained in each of these Examples was approximately equal indiameter and length to that of Example 1.

TABLE 1 Catalyst Atmosphere Amount Pressure Temperature Example Type(mg) Type (Pa) (° C.) 4 FeCl₂ 10 Vacuum 3 650 5 FeCl₂ 10 He 65800 800 6MgCl₂ 10 Vacuum 3 800 7 FeCl₂ 10 Ar 750 700 8 FeCl₂ 10 Nitrogen 750 7009 FeCl₂ 10 Vacuum 3 1500 10 FeCl₂ 10 Nitrogen 2 × 10⁵ 900

FIG. 5 is an X-ray diffraction chart of the carbon tubes obtained inExample 9. In the product obtained by treatment at a relatively hightemperature of 1500° C. in Example 9, the X-ray diffraction angle (2θ)was 19.1 degrees, the spacing between the hexagonal carbon layers (d₀₀₂)calculated from the X-ray diffraction angle was 4.6 Å, and the 2θ bandhalf-width was 8.1 degrees.

EXAMPLE 11

0.6 g of an anhydrous magnesium chloride hexahydrate powder (having aparticle size not greater than 500 μm) and 0.3 g of an anhydrous lithiumchloride powder were dusted uniformly over 1 g of a PTFE film with athickness of 60 μm. The film was placed in a vacuum furnace. The furnacewas purged with nitrogen 3 times, and the pressure in the furnace wasreduced to 0.1 Pa. Then, the film was vacuum heated at 900° C. for 30minutes.

SEM and X-ray diffraction confirmed formation of amorphous nano-scalecarbon tubes having a diameter of 10 to 60 nm and a length of 5 to 6 μm.

EXAMPLE 12

10 g of a polyvinyl alcohol powder and 10 mg of FeCl₂ were mixedtogether and placed in a vacuum furnace. The furnace was purged withnitrogen 3 times. The pressure in the furnace was reduced to 0.1 Pa, andthen the mixture was vacuum heated at 900° C. for 30 minutes.

SEM and X-ray diffraction confirmed formation of amorphous nano-scalecarbon tubes having a diameter of 10 to 60 nm and a length of 5 to 10μm.

EXAMPLE 13

10 g of a polyethylene powder and 10 mg of FeCl₂ were mixed together andplaced in a vacuum furnace. The furnace was purged with nitrogen 3times. The pressure in the furnace was reduced to 0.1 Pa, and then themixture was vacuum heated at 900° C. for 30 minutes.

SEM and X-ray diffraction confirmed formation of amorphous nano-scalecarbon tubes having a diameter of 10 to 60 nm and a length of 5 to 10μm.

EXAMPLE 14

10 g of a polyimide powder and 10 mg of FeCl₂ were mixed together andplaced in a vacuum furnace. The furnace was purged with nitrogen 3times. The pressure in the furnace was reduced to 0.1 Pa, and then themixture was vacuum heated at 900° C. for 30 minutes.

SEM and X-ray diffraction confirmed formation of amorphous nano-scalecarbon tubes having a diameter of 10 to 60 nm and a length of 5 to 10μm.

EXAMPLE 15

10 g of a polyacrylonitrile powder and 10 mg of FeCl₂ were mixedtogether and placed in a vacuum furnace. The furnace was purged withnitrogen 3 times. The pressure in the furnace was reduced to 0.1 Pa, andthen the mixture was vacuum heated at 900° C. for 30 minutes.

SEM and X-ray diffraction confirmed formation of amorphous nano-scalecarbon tubes having a diameter of 10 to 60 nm and a length of 5 to 10μm.

REFERENCE EXAMPLE 1

PTFE films were electrolytically or chemically reduced to synthesizecarbon materials containing —C≡C— and/or ═C═ in their surface layers.

(1) Electrolytic reduction was performed by the two electrode method(anode: magnesium, cathode: stainless steel) using, as a solvent, atetrahydrofuran solution of indicator salts (LiCl: 0.8 g, FeCl₂: 0.48 g,THF: 30 ml). 10 PTFE films (10 mm×10 mm×0.03 mm) and the solvent wereplaced in a flask equipped with an anode and a cathode, followed byelectrolytic reduction with stirring in an argon atmosphere at 0° C. for15 hours. During the reaction, a potential of 25 V was applied betweenthe anode and the cathode. After completion of the reaction, the PTFEfilms were washed with THF, vacuum dried and stored in an argonatmosphere.

TEM observation of the cross sections of the films revealed that thesurface portion of each film had been reduced and converted into a 10μm-thick layer of a carbon material. Raman spectrum exhibited a band at2100 cm⁻¹ attributable to C≡C, and a band at 1500 cm⁻¹ attributable toC═C. The above observation results demonstrate that, in the obtainedmaterial, the surface layers of the PTFE films had been converted into acarbon material containing —C≡C— and/or ═C═.

(2) Chemical reduction was carried out as follows: 10.0 g of particulateMg, 2.66 g of anhydrous lithium chloride (LiCl), 1.60 g of anhydrousferrous chloride (FeCl₂), 20 PTFE films (8 mm×8 mm×50 μm, total weight:about 0.2 g) and a stirring bar were placed in an egg plant type flask(hereinafter referred to as “the reactor”) with an internal volume of100 ml equipped with a three-way cock. These raw materials were dried at50° C. under a reduced pressure of 1 mmHg. Then, a dry argon gas wasintroduced into the reactor, and 44 ml of THF previously dried withsodium-benzophenone ketyn, followed by stirring at room temperature forabout 3 hours using a magnetic stirrer.

After completion of stirring, the PTFE films, which had been convertedto black-colored carbonaceous material, were recovered from the reactionmixture, washed twice with 20 ml of dry THF, and vacuum dried. TEMobservation of the cross sections of the films revealed that a 10μm-thick surface layer of each film had been reduced and converted intoa carbon material. Analysis with a Raman spectrometer clearly showed apeak attributable to C═C (1500 cm⁻¹) and a peak attributable to C≡C(2100 cm⁻¹), which had not been observed in the original PTFE films.

The following Examples were carried out using samples synthesized by theabove process (1) and those synthesized by the process (2), unlessotherwise specified.

EXAMPLES 16 TO 20

The procedure of Reference Example 1 was followed to form a 10 μm-thickcarbon material having —C≡C— and/or ═C═ structure of PTFE films. Thecarbon material was heat-treated under the conditions shown in Table 2.

TABLE 2 Atmosphere Pressure Temperature Example Type (Pa) (° C.) 16Vacuum 3.5 600 17 Vacuum 3.5 800 18 Vacuum 3.5 900 19 Vacuum 3.5 1100 20Vacuum 3.5 1500

SEM and X-ray diffraction observation confirmed that a large amount ofamorphous nano-scale carbon tubes (diameter: 10 to 60 nm, length: 5 to 6μm) was formed at any of the above temperatures.

TEM photographs of the product obtained in Example 18 are shown in FIG.6-A and in FIG. 6-B (an enlarged fragmentary view of the product shownin FIG. 6-A).

As to the carbon nanotubes of the above Examples, it was confirmed byTEM observation that the dimension in the planar direction of thehexagonal carbon layers is smaller than the diameter of the carbonnanotubes.

Table 3 presents X-ray diffraction data of typical samples. The X-raydiffraction angle (2θ) was in the range of 18.9 to 22.6 degrees; thespacing between the hexagonal carbon layers (d₀₀₂) was in the range of3.9 to 4.7 Å; and the 2θ half-width of the was in the range of 7.6 to8.2 degrees. Thus, the carbon material of the invention was confirmed tohave an amorphous structure.

TABLE 3 Diffraction d₀₀₂ diffraction 2θ band half- angle band widthExample (2θ; degree) (Å) (degree) 1 19.1 4.6 8.1 3 18.9 4.7 8.2 5 18.94.7 8.2 6 18.9 4.7 8.2 9 19.1 4.6 8.1 18 20.0 4.4 8.0 20 22.6 3.9 7.6

SEM and TEM observation revealed that the amorphous nano-scale carbontubes obtained in the Examples had a straight shape, and were notentangled with one another.

COMPARATIVE EXAMPLE 1

Carbon nanotubes were synthesized by a conventional arc dischargemethod, purified, and observed with SEM, TEM and X-ray diffraction.

SEM and TEM observation showed that the carbon nanotubes obtained inComparative Example 1 had a graphitic structure, had a curvilinearshape, and were entangled with one another. The X-ray diffraction angle(2θ) was 26.2 degrees, the spacing between the hexagonal carbon layers(d₀₀₂) was 3.40 Å, and the 2θ band half-width was 0.9 degree. Theseresults also confirmed that the carbon nanotubes had a graphiticstructure.

The amorphous nano-scale carbon tubes obtained in the Examples 1 weretested for repetitive hydrogen-storing properties at 100 atms by agravimetric method (with buoyancy correction) using a two-componentadsorption measurement apparatus (“FMS-BI-H” manufactured by BEL JAPAN,INC.).

Table 4 shows the hydrogen-storing properties of typical samples. Aftera hydrogen-releasing step, the amount of adsorbed hydrogen in eachmaterial became substantially zero. It was confirmed by the resultsshown in Table 4 that the amorphous nano-scale carbon tubes of theinvention have a high capacity for storing hydrogen, and highdurability.

TABLE 4 Amount of hydrogen stored (wt. %) 1st 3rd 5th 10th 20th 30thtrial trial trial trial trial trial Example 3 1.6 1.6 1.6 1.6 1.6 1.6Example 9 2.3 2.3 2.3 2.3 2.3 2.3 Comp. Ex. 1 0.2 0.2 0.2 0.1 0.0 0.0

It was also confirmed that the carbon materials obtained in the Exampleshave a high capacity for storing other gases (such as methane, helium,neon, xenon, krypton and carbon dioxide), which is equivalent to theirhydrogen-storing capacity.

1. Amorphous nano-scale carbon tubes each containing a main frameworkwhich comprises carbon, and each having a straight shape, a diameter of0.1 to 1000 nm and an amorphous structure, and each having an interlayerspacing (002) between hexagonal carbon layers of at least 3.7 Å, adiffraction angle (2θ) of 24.1 degrees or less, and a 2θ degrees orless, and a 2θ band half-width of at least 3.2 degrees, as determinedwith a diffractometer by an X-ray diffraction method (incident X-Ray:CuKα).
 2. Amorphous nano-scale carbon tubes according to claim 1, eachof which comprises hexagonal carbon layers each having a dimension ofthe planar direction that is smaller than the diameter of the carbontube, as determined from a transmission electron microscope image. 3.Amorphous nano-scale carbon tubes according to claim 1, each of whichhas a 2θ band half-width of at least 7.0 degrees, as determined with adiffractomer by an X-ray diffraction method (incident X-ray: CuKα). 4.The amorphous nano-scale carbon tubes according to claim 1, each ofwhich has an interlayer spacing (002) between hexagonal carbon layers of3.9 to 4.7 Å, a diffraction angle (2θ) of 18.9 to 22.6 degrees, and a 2θband half-width of 7.6 to 8.2 degrees, as determined with adiffractometer by an X-ray diffraction method (incident X-ray: CuKα). 5.Amorphous nano-scale carbon tubes according to claim 1, each of whichhas a hollow cylindrical shape or a hollow rectangular prism shape. 6.Amorphous nano-scale carbon tubes according to claim 1, each of whichhas at least one open end.
 7. Amorphous nano-scale carbon tubesaccording to claim 1, which are formed on a substrate, a particle or aporous material.
 8. A gas-storing material comprising an amorphouscarbonaceous material containing the amorphous nano-scale carbon tubesaccording to claim
 1. 9. The gas-storing material according to claim 8,which contains at least one of a metal salt and a metal.
 10. Thegas-storing material according to claim 9, wherein the metal salt andthe metal are selected from the group consisting of iron, cobalt,nickel, copper, platinum, palladium, rubidium, strontium, cesium,vanadium, manganese, aluminum, silver, lithium, potassium, sodium,magnesium, hydrogen-occluding alloys and metal complexes.
 11. A methodfor storing a gas, wherein a gas is stored using the gas-storingmaterial according to any one of claims 8 to
 10. 12. The methodaccording to claim 11, wherein the gas to be stored is hydrogen,methane, helium, neon, xenon, krypton or carbon dioxide.
 13. A methodfor producing a carbon material containing amorphous nano-scale carbontubes according to claim 1, the method comprising subjecting a heatdecomposable resin having a decomposition temperature of 200 to 900° C.to excitation treatment in the presence of a catalyst, the heatdecomposable resin being selected from the group consisting ofpolytetrafluoroethylene, polyvinylidene chloride, polyvinylidenefluoride and polyvinyl alcohol, and the catalyst being at least onehalide of a metal selected from the group consisting of magnesium, iron,cobalt and nickel.
 14. The method according to claim 13, wherein thecatalyst is iron chloride.
 15. The method for producing said carbonmaterial containing the amorphous nano-scale carbon tubes according toclaim 13 or 14, wherein the excitation treatment of the heatdecomposable resin is carried out by a heat treatment in an inert gas ata temperature of 300 to 3000° C.
 16. The method for producing saidcarbon material containing the amorphous nano-scale carbon tubesaccording to claim 13 or 14, wherein the excitation treatment of theheat decomposable resin is carried out by a light irradiation treatmentin an inert gas at a temperature of room temperature to 3000° c.
 17. Themethod for producing said carbon material containing the amorphousnano-scale carbon tubes according to claim 13 or 14, wherein theexcitation treatment of the heat decomposable resin is carried out byplasma treatment in an inert gas at a temperature of room temperature to3000° C.
 18. The method for producing said carbon material containingthe amorphous nano-scale carbon tubes according to claim 13 or 14,wherein the excitation treatment of the heat decomposable resin iscarried out by electron beam irradiation treatment in an inert gas at atemperature of room temperature to 3000° C.
 19. The method for producingsaid carbon material containing the amorphous nano-scale carbon tubesaccording to claim 13 or 14, wherein the excitation treatment of theheat decomposable resin is carried out by ion beam irradiation treatmentin an inert gas at a temperature of room temperature to 3000° C.
 20. Acarbon material containing the amorphous nano-scale carbon tubesaccording to claim 1.